A Portable electric apparatus such as cellular phones, personal digital assistants (PDA), portable digital versatile discs (DVDs), camcorders, digital cameras, etc., have a battery built therein for the purpose of using them when a user moves from a place to another. The portable electric apparatus employs a built-in battery in case of no an AC power supply. Such a built-in battery is used during a predetermined period of time. Thus, when the use time of the built-in battery exceeds the predetermined time period, the battery cannot be used any more.
A first method for addressing and solving this problem includes a provision of an external battery charger for recharging a spare battery. The external battery charger has a shortcoming in that it cannot be used in the case where there is no external power supply such as a home AC or DC power supply.
Secondly, if there is no the external power supply, an external battery can be used. A general external battery pack with a built-in secondary battery has a merit in that it can be used even in case of no electric power supply since it is internally attached with has a charge circuit and a DC/DC circuit fit for the power supply of an electronic device to be used.
However, such an external battery pack has a demerit in that it is expensive, and in that a charged battery therein cannot be replaced for other purpose since a pack battery is securely fixed thereto.
A third method for addressing and solving the above problem includes a provision of an emergency power supply employing a first battery and a secondary battery. This emergency power supply has an advantage in that it can utilize the first and secondary battery, since it is internally attached with a DC/DC circuit fit for the power supply of an electronic device, but not with a charge circuit. However, the emergency power supply has a disadvantage in that it cannot charge the secondary battery that has been used already.